1. Field of the Invention
This invention relates to a method for making polyvinylidene fluoride (PVDF) films having an enhanced piezoelectric constant. More particularly, the invention encompasses rolling a PVDF film that has previously been stretched in order to obtain a preferred crystalline orientation.
2. Description of the Prior Art
In the prior art, polarized PVDF films are known to have a relatively high piezoelectric constant. This makes polarized PVDF films suitable for use as electromechanical transducers. These include microphones, pressure gauges, pressure activated switches, etc., as well as transducers employing the inverse piezoelectric effect, such as speakers, etc. Polarized PVDF films also have a relatively large pyroelectric constant, and they can be used to frequency-double laser light.
In order to enhance the piezoelectric constant, the prior art teaches the use of mechanical deformation techniques. In particular, U.S. Pat. No. 3,931,446 discloses a technique whereby a PVDF film having a predominantly .alpha.-type crystal structure is stretched in order to obtain a film having a predominantly .beta.-type crystal structure. That technique, preferentially accomplished at an elevated temperature, increases the piezoelectric constant over unstretched films after the film is poled in an electric field. It is also known that rolling a film may increase the percentage of the .beta.-type crystal structure in the film; see "Piezoelectricity, Pyroelectricity, and the Electrostriction Constant of Poly(vinylidene Fluoride)," K. Nakamura et al, Journal of Polymer Science, Part A-2, Vol. 9, pp. 161-173 (1971). However, the rolling technique is limited to a film elongation ratio of about 2, which is less than can be achieved by the stretching technique, implying a greater percentage of .beta. crystal structure in highly stretched films, and a higher piezoelectric constant.
A conversion from .alpha.-type to .beta.-type structure can also be accomplished in PVDF films by poling in a high electric field; see "Electric-field-induced Phase Changes in Poly(vinylidene fluoride)," G. T. Davis et at, Journal of Applied Physics, 49 (10), pp. 4998-5002 (1978).
It is also known in the prior art that PVDF films are more readily polarized by poling in an electric field at elevated temperatures. Furthermore, it is generally known thermodynamically that an increase in the piezoelectric constant is often accompanied by an increase in the pyroelectric constant.
It is generally the objective of this invention to further improve the piezoelectric, inverse piezoelectric, and pyroelectric effects in PVDF films, for use in a wide variety of devices.